Field of the Invention
This invention pertains generally to the field of aircraft display units that display images acquired by forward-looking image capturing devices.
Description of the Related Art
It is well-known that a Synthetic Vision System (“SVS”) may generate image data representative of a synthetic, three-dimensional perspective of a heading-based scene in front of the aircraft. When provided to a display system, the synthetic image could be presented on the screen of a display unit. Also, it is well-known that an Enhanced Vision System (“EVS”) may generate image data representative of the real-world as an enhanced image of the heading-based scene in front of the aircraft, where such image data has been acquired by one or more fixed or steerable image capture devices. When provided to the display system, the enhanced image could be presented on the screen of the display unit. Also, as disclosed by Wenger et al in U.S. Pat. No. 7,605,716, image data from both the SVS and EVS may be combined to form a combined SVS-EVS image that could be presented on the display unit.
Unlike the heading-based synthetic image, a track-based synthetic image (i.e., a synthetic image based on the ground track of the aircraft and not the heading) may have an operational advantage when used in conjunction with Highway-in-the-Sky (“HITS”) symbology. If the aircraft is flying within the HITS, the will be centered within the track-based synthetic image. The use of the track-based synthetic image could also prevent the unwanted intrusion of the HITS and FPV symbologies into other symbologies (e.g., a speed tape or an altitude tape) when strong crosswinds are present.
The EVS may employ fixed forward-looking image capture devices that are oriented with the heading of the aircraft. When a heading-based SVS image is combined with a heading-based EVS image, there is no misalignment of the images due to crosswinds. When a track-based image is combined with a heading-based image, there is no misalignment of the images if there is no crosswind; however, in the presence of a crosswind, the alignment between the two images may be lost because a track-based image is referenced to the ground track and the heading-based image is referenced to the heading. As such, there will be a misalignment between the two images resulting in an inconsistent and invalid representation of the track-based scene in front of the aircraft.
The ability to position a heading-based image within a track-based image to correct for the misalignment has been disclosed by Barber in U.S. Pat. No. 8,831,798 entitled “Systems and Methods for Positioning a Heading-Based Image within a Track-Based Image and for Generating Steering Commands to a Steerable Forward-Looking Image Capture Device of an Enhanced Vision System,” where wind correction angle (“WCA”) information was employed in the positioning of the heading-based image within a track-based image.
As stated above, there is no misalignment of the images on the occasion where there is no crosswind. There is a second occasion for which there is no misalignment, and this occurs when the aircraft is on the ground, where ground track and heading are the same. Although the absence of a misalignment of images is favorable, it is not helpful on the occasion when an aircraft is taxiing under low visibility conditions and under which the EVS may be employed to generate an image representative of the scene outside the aircraft that is not visible to a pilot; however, with fixed forward-looking image capture devices, the image is limited to the scene straight ahead of the aircraft. If an aircraft is approaching a taxiway intersection and other aircraft are approaching the same intersection from a different direction(s), the image may not capture the other aircraft if the other aircraft are beyond the field of view of the image capture device(s).